An MRI apparatus is designed to obtain magnetic resonance images (hereinafter referred to as MR images) that represent the physical property of a subject, when irradiating electromagnetic waves to a subject placed in an homogeneous static magnetic field, by using the nuclear magnetic resonance phenomena being generated in the nucleus of the atoms that compose the subject, detecting the nuclear magnetic resonance signals (hereinafter referred to as NMR signals) from the subject, and reconstructing the images by these NMR signals. In order to give the positional information of this imaging, a gradient magnetic field is applied being superposed over a static magnetic field.
In an MRI apparatus, when the ferromagnetic materials such as permanent magnets are used as the source of static magnetic field, the residual magnetic field accompanied by hysteresis phenomena is generated by the application of the above-mentioned gradient magnetic field. This residual magnetic field is generated by residual magnification caused by things such as a permanent magnet, being ferromagnetic material. Even when the application of the gradient magnetic field is stopped, the magnetic field with the strength and direction according to its application history still remains. This, along with eddy currents being generated in the structure by the application of the gradient magnetic field, complicates the magnetic field of the imaging space and causes the deterioration of the image quality.
A conventional technique for removing the affect that the residual magnetic field gives to the MRI apparatus is described in Patent Document 1.
Patent Document 1: JP-A-2000-157509
However, even though the conventional technique described in Patent Document 1, in relation to the test gradient magnetic field, obtains a correction quantity of the gradient magnetic field by measuring the residual magnetic field in the form of tables and functions, the application history thereof is not taken into consideration.